MC-CDMA systems and spread OFDMA systems are candidate systems for third generation partnership project (3GPP) long term evolution that can support a very high data rate for both downlink and uplink transmissions.
MC-CDMA systems are direct spreading code division multiple access (DS-CDMA) systems wherein the spreading is performed in a frequency domain. At the transmitter, N data symbols of each user generated in each OFDM symbol duration are spread into NM chips using a length-M spreading code and mapped to multiple subcarriers. The chips from different users are then added together and the resulting chips are fed to an inverse Fourier transform processor of an OFDM transmitter. The data symbols of each user can be recovered by despreading the output of a Fourier transform processor in a receiver.
OFDMA is an alternative multiple access scheme for OFDM systems, where the signals of different users are separated in the frequency domain by allocating different sub-carriers to different users.
When MC-CDMA or spread OFDMA is used for downlink transmissions, one tap equalizer with despreading across subcarriers may be sufficient for data detection when used subcarriers have the same or very similar channel responses because all the users have the same propagation channel and spreading codes are orthogonal across different subcarriers. However, the subcarriers often have different channel responses, which destroys the orthogonality of codes in the despreading processes, and one-tap matched filter or equalizer may not be sufficient for data detection. When MC-CDMA or spread OFDMA is used for uplink transmissions, one tap equalizer suffers significant performance degradation because users have distinct propagation channels in the uplink and each user experiences different delay spread and fading channel and, therefore, the orthogonality between spreading codes is destroyed.